The *A round the world doesnt have backtracking rules (other than between IATA regions), so you can continue flying around USA when you get back. You will need to:

stay within US/Canada (once you return to country of origin cannot leave it again and Canada is counted as same country as US for the purpose of this rule)

stay within your total mileage limit

make sure no more than 3 transfers at any one city

make sure no more than 1 stopover at any one city

stay within 24 total flights on the ticket

finish within 12 months

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So with lots of mileage up your sleeve you could do a couple of mileage runs on the RTW ticket. If possible book on 2-class domestic flights so you get the first class service bonus. Assuming you are crediting to UA then some short flights are also good (for the minimum mileage earning).